Fix #4601: Hide deleted pools by default in pool search.

* On /pools, hide deleted pools by default in HTML responses. Don't
  filter out deleted pools in API responses.

* API change: on /forum_topics, only hide deleted forum topics by
  default for HTML responses, not for API responses. Explicitly do
  https://danbooru.donmai.us/forum_topics.json?search[is_deleted]=false
  to filter out deleted topics.

* API change: on /tags, only hide empty tags by default for HTML
  responses, not for API responses. Explicitly do
  https://danbooru.donmai.us/tags.json?search[is_empty]=false to filter
  out empty tags.

* API change: on /pools, default to 20 posts per page for API responses,
  not 40.

* API change: add `search[is_empty]` param to /tags.json endpoint.
  `search[hide_empty]=true` is deprecated in favor of `search[is_empty]=false`.

* On /pools, add option to show/hide deleted pools in search form.

* Fix the /forum_topics page putting `search[order]=sticky&limit=40` in
  the URL when browsing past page 1.

app/controllers/forum_topics_controller.rb

@@ -15,11 +15,12 @@ class ForumTopicsController < ApplicationController
   end
 
   def index
-    params[:search] ||= {}
-    params[:search][:order] ||= "sticky" if request.format.html?
-    params[:limit] ||= 40
-
-    @forum_topics = authorize ForumTopic.visible(CurrentUser.user).paginated_search(params)
+    if request.format.html?
+      limit = params.fetch(:limit, 40)
+      @forum_topics = authorize ForumTopic.visible(CurrentUser.user).paginated_search(params, limit: limit, defaults: { order: "sticky", is_deleted: false })
+    else
+      @forum_topics = authorize ForumTopic.visible(CurrentUser.user).paginated_search(params)
+    end
 
     if request.format.atom?
       @forum_topics = @forum_topics.includes(:creator, :original_post)

app/controllers/pools_controller.rb

@@ -12,7 +12,11 @@ class PoolsController < ApplicationController
   end
 
   def index
-    @pools = authorize Pool.paginated_search(params, count_pages: true)
+    if request.format.html?
+      @pools = authorize Pool.paginated_search(params, count_pages: true, defaults: { is_deleted: false })
+    else
+      @pools = authorize Pool.paginated_search(params, count_pages: true)
+    end
 
     respond_with(@pools)
   end

app/controllers/tags_controller.rb

@@ -7,7 +7,12 @@ class TagsController < ApplicationController
   end
 
   def index
-    @tags = authorize Tag.paginated_search(params, hide_empty: true)
+    if request.format.html?
+      @tags = authorize Tag.paginated_search(params, defaults: { hide_empty: true })
+    else
+      @tags = authorize Tag.paginated_search(params)
+    end
+
     @tags = @tags.includes(:consequent_aliases) if request.format.html?
     respond_with(@tags)
   end